EGLstream backend fixes

Merged Olivier Fourdan requested to merge ofourdan/xserver:issue1156 into master

This is a merge request does three things:

  1. Check the buffer is a thing prior to attaching it to the Wayland surface in the hope to avoid a Wayland protocol error or some other crash,
  2. If glamor detects a GL error, log the error along with a backtrace in the hope that it would give what caused the error.
  3. Avoid the GL error, caused by a wrong stream state following the disposal of the stream while the compositor is still using it.

This is to help (possibly) with #1156 (closed)

With this, as it is, there is no more GL error and no more abort due to a Wayland error caused by attaching a wrong buffer, yet there are visual glitches remaining, but this is a separate issue.

Edited by Olivier Fourdan

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